Seven projects are contained under a Program Project aiming at the understanding, at the biochemical and molecular level, of processes leading to mental retardation. Comprehensive titles of the projects describe adequately the broad aims of the program. I. Identification and Study of Metabolic Diseases in Relation to Mental Retardation. II. Experimental and Clinical Study of the Effects of Mercurials on Neural Metabolism and Development. III. Disorders of Energy Metabolism Associated with Neurological Diseases. IV. Molecular Biology of Metachromatic Leukodystrophy and Transferase Deficiency Galactosemia. V. Characterization of the Genetic Lesion in Phenylketonuria. VI. Molecular Biology of Phenylalanine Hydroxylase Gene. Cell Culture Studies. VII. Role of Ketone Bodies in Brain Metabolism Especially During Myelination.